The present invention relates to an atomizer wheel for the atomization of slurries of abrasive material, comprising a wheel hub and a mainly cylindrical external wall defining an annular chamber of a substantially bowl-like cross-sectional shape coaxially surrounding said hub, a plurality of ejection apertures, each extending in a substantially radial direction, distributed over the circumference of said external wall, and a plurality of inserts arranged in said ejection apertures, each insert having a longitudinal axis extending in said substantially radial direction and comprising a bushing, the external side of which is fitting into said external wall, and a wear-resistant lining arranged in said bushing such that a contact surface of said lining abuts against an internal side of said bushing, the inner end of each of said inserts projecting into said annular chamber.
The invention further relates to an insert for use in such a wheel.
During atomization of a slurry such inserts are exposed to wear which, depending on the material to be atomized, may even be very heavy. This wear is normally restricted, however, to certain well defined areas and for that reason the bushings/linings are preferably mounted so that they can be turned as they are gradually being worn in order to increase their lifetime.
A number of precautions have been taken in order to prevent deformation and fracture of the inserts, eg. as disclosed in U.S. Pat. Nos. 3,640,467 and 4,684,065 (Niro).
In both of these documents the inserts project a distance into the annular space in the wheel in order to ensure that a layer of liquid will deposit on the external wall during the rotation of the wheel and hereby prevent wear. Owing to the centrifugal forces the heavier parts of the layer, i.e. the suspended solid material, is separated from the liquid in the layer and deposited directly on the wall so as to fill the area surrounding the inwardly extending portions of the inserts.
The principle of the abrasion resistant atomizer wheel is as follows:
Parts exposed to abrasion from feed are abrasion resistant and replaceable. The inner surface of the wheel rim which is also exposed to contact with the feed is protected against abrasion by the unique principle of inwards-protruding inserts. During operation a layer of feed solids will settle on the inside wall to a thickness determined by the length of the protrusion, consequently the abrasion will take place on the sedimented layer itself and not on the inside wall of the wheel.
It has turned out, however, that during atomization of some abrasive slurries, the foremost part of the inserts, seen in the tangential or rotational direction, is exposed to an extremely high wear which has entailed that not only the wear-resistant lining but also the steel bushing show sign of wear. It is believed that this effect in due to the fact that whirl formation prevents the formation of a protective layer locally at the front end of the inserts. The consequences are that the inserts have to be changed frequently in order to prevent wear on the atomizer wheel itself, which could lead to damage of the circumferential external wall of the wheel and eventually breakdown of the atomizer wheel.
With this background it is an object of the present invention to provide an atomizer wheel which is able to perform atomization of even very abrasive material without the need for frequent replacement of the inserts and in which the risk of damage of the wheel itself is almost eliminated.
This object is met by an atomizer wheel of the initially stated kind, which is characterized in that each insert is provided with a transition surface extending between an inner end face of the lining and said contact surface, and that said transition surface comprises at least one portion forming an angle with said contact surface.
By this design it is possible to direct the flow of the material to be atomized along the transition surface of the lining and further down to the inner side layer on the external wall and thus avoid wear on the bushing. Without intending to limit the scope of the invention to a specific explanation, it is believed that the whirl which is formed in front of the inserts in traditional atomizer wheels is controlled as to direction to such an extent that it is now possible to form a coherent layer of protective material on the inner side of the external wall and that any remaining detrimental effects of such a whirl only effect the lining and not the bushing.
Furthermore, it has turned out that the vibrations of the drive shaft in an atomizer wheel according to the invention are reduced, apparently due to a more controlled flow in the atomizer wheel.
In a preferred embodiment, the transition surface is substantially concave, by which it is secured that the transition surface leads the flow past the bushing by imparting on the slurry to be atomized a substantial change of directions.
In another aspect of the invention, an insert for an atomizer wheel is provided, said insert having a longitudinal axis and comprising a bushing and a wear-resistant lining arranged in said bushing such that a contact surface of said lining abuts against an internal side of said bushing. The insert is characterized in that said insert is provided with a transition surface extending between an inner end face of the lining and said contact surface, and that said transition surface comprises at least one portion forming an angle with said contact surface.
Further advantageous embodiments of the invention are the subject of the remaining dependent claims.
In the following the invention will be described in further detail by means of an embodiment thereof and the appended drawings.